Measurement of rare decays of the b quark

Using a data sample of 2.6 million BB pairs produced at the ϒ(4S) taken with the CLEO-II detector at the Cornell Electron Storage Ring (CESR) we have searched for 2-body rare hadronic and semileptonic decays of the B-meson, involving the b → u transition. We measure the branching fraction of the sum of B0 → π+π− and B0 → K+π− to be (1.8−0.5−0.3+0.6+0.2 ± 0.2) × 10−5 and place upper limits in the range from 10−4 to 10−6 on other rare hadronic decays. For exclusive semileptonic b → u processes (using the ISGW model for efficiencies), we find braching ratios B(B0 → π−ℓ+ν) = (1.34 ± 0.35 ± 0.28) × 10−4, and B(B0 → ϱ−ℓ+ν) = (2.28±0.36±0.59−0.46+0.00) × 10−4 and the ratio of widths Γ(B0 → ϱ−ℓ+ν)Γ(B0 → π−ℓ+ν) = 1.70−0.05+0.81±0.58−0.34+0.00. If instead of ISGW one uses the WSB model, one finds B(B0 → π−ℓ+ν) = (1.63 ± 0.46 ± 0.34) × 10−4, B(B0 → ϱ−ℓ+ν) = (3.88±0.54±1.01−0.78)+0.00 × 10−4, and Γ(B0 → ϱ−ℓ+ν)Γ(B0 → π−ℓ+ν) = 2.39−0.69+1.18±0.81−0.48+0.00.